1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new sandwich cathode design having a first cathode active material of a relatively low energy density but of a relatively high rate capability sandwiched between two current collectors and with a second cathode active material having a relatively high energy density but of a relatively low rate capability in contact with the opposite side of the current collectors. The present cathode design is useful in applications where a premium is places on increased energy density, such as in power sources associated with implantable medical devices, while providing relative safety under short circuit conditions.
2. Prior Art
The capacity of an electrochemical cell is not only dependent on the electrode assembly design and packing efficiency, it also is dependent on the type of active materials used. For example, it is generally recognized that for lithium cells, silver vanadium oxide (SVO) and, in particular, xcex5-phase silver vanadium oxide (AgV2O5.5), is preferred as the cathode active material. This active material has a theoretical volumetric capacity of 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of CFx material (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of xcex5-phase silver vanadium oxide.
An attempt to use high capacity materials, such as CFx, by mixing it with a high rate cathode material, such as SVO, is reported in U.S. Pat. No. 5,180,642 to Weiss et. al. However, electrochemical cells made from such cathode composites have lower rate capability. Increasing the cell""s theoretical capacity by using CFx as part of the cathode mix is in part canceled by lowering of its power capability in a high rate discharge application.
U.S. Pat. No. 5,614,331 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated hereby by reference, describes a method of using a medium rate CFx cell to power the circuitry of an implantable defibrillator while simultaneously using a SVO cell to provide the power supply under high rate application for the device. The advantage of this method is that all of the high power SVO energy is reserved for the high power application such as charging a capacitor while the device monitoring function, for example for monitoring the heart beat, which require generally low power requirements, is provided by the high capacity CFx system. However, this battery construction requires a very careful design to balance the capacities of the high power cell (SVO) and the low power cell (CFx) with both cells reaching end of service life at or near the same time. Such a balance, nevertheless, is very difficult to achieve due to the variable device usage requirements of a particular patient.
Accordingly, the present invention is directed to improvements in the performance of lithium electrochemical cells by providing a new concept in electrode design. The new electrode configuration is especially useful in applications where increased energy density is desired while providing relative safety under short circuit conditions. Also, the cell has a predictable end of life, which is useful for scheduling cell replacement procedures, such as in implantable medical device applications.
To fulfil these needs, a new sandwich cathode design is provided having a first cathode active material of a relatively low energy density but of a relatively high rate capability, for example SVO, sandwiched between two current collectors and with a second cathode active material having a relatively high energy density but of a relatively low rate capability, for example CFx, in contact with the opposite sides of the current collectors. Such an exemplary cathode design might look like: CFx/current collector/SVO/current collector/CFx.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.